From yeast or pancreas. Prepn: P. A. Levene, L. W. Bass, Nucleic Acids (New York, 1931) pp 224-227. Structure: Levene, Jorpes, J. Biol. Chem. 81, 579 (1929); Levene, Harris, ibid. 95, 755 (1932); 98, 9 (1932). Early work probably done on a mixture of 2′- and 3′-guanylic acids; see physical data below. Separation of two isomers: Cohn, J. Am. Chem. Soc. 72, 1471 (1950); Khym, Cohn, ibid. 76, 1818 (1954); eidem, Biol. Prepn. 5, 40 (1957). Absorption spectrum: Voet et al., Biopolymers 1, 193 (1963). Reviews: see Guanine.